posted by matt .
A certain car's cooling system has an 8-qt capacity and is filled with a mixture that is 30% alcohol. How much of this mixture must be drained off and replaced with pure alcohol if the solution is to be 50% alcohol?
the amount of alcohol in the parts must add up to the whole. So,
.3 * (8-x) + 1.0 * x = .5*8
x = 16/7